This invention relates to the hot rolling and direct sequential cooling of steel rod. As herein employed, the term "rod" is used to designate a product ranging from about 4.0 to 8.0 mm. in diameter.
Conventionally, steel rod exits from the mill finishing train at temperatures of at least about 1038.degree. C. The rod proceeds via delivery pipes directly from the mill finishing train through water boxes where it is cooled by a surface application of cooling water. Thereafter, the rod is directed to a laying head where it is formed into a succession of rings. The rings are normally deposited in an offset or Spencerian pattern on an open moving conveyor, where they are subjected to additional controlled cooling before finally being accumulated into coils.
Due to the relatively high temperatures at which the rod is finish rolled, it has very little if any column strength as it exits from the mill. In modern high speed mills, i.e., those having finishing speeds of at least about 75 m./sec., this severely limits the extent to which the rod can be cooled in the water boxes as it travels from the mill to the laying head. This limitation stems from the fact that there is a frictional resistance imposed on the rod by the cooling water. If this frictional resistance is allowed to exceed what little column strength the rod has, then the rod will collapse or "cobble". This problem becomes increasingly acute as rod diameters decrease and mill delivery speeds increase. Thus, in conventional high speed mills, depending on the size of the product being rolled and the mill delivery speed, the minimum temperatures to which rod cans safely be water cooled before being laid on the conveyors usually range from about 760.degree. C. to 927.degree. C.
As a further precautionary measure in avoiding cobbles, it has become customary in high speed mills not to begin water cooling the rod until after its front end has passed through the water boxes and the laying head and rings have begun to accumulate on the conveyor. The uncooled front section of the rod thus lacks the desired metallurgical structure which results at least in part from water cooling. The front section must, therefore, be scrapped. Such scrap loses can be considerable, in some cases amounting to as much as 0.6% of the mill's annual production.
Against this backdrop, there is now a growing interest in processes which involve subjecting hot rolled steel rod to a much more drastic water quench, thereby enabling the rod to be laid on the conveyor at temperatures well below 760.degree. C. Among the objectives of such processes are the reduction of scale formation on the rod surface and the production of specific microstructures and mechanical properties. U.S. Pat. No. 3,926,689 discloses one such process where the product exiting from the mill is rapidly quenched to provide a surface layer of bainite or martensite which is then tempered by the heat transferred from the product core to its surface during subsequent cooling. In order to achieve this result, a rapid surface quenching is required down to about 300.degree. C. Such processes have been employed successfully in bar mills, where products having diameters larger than about 14.0 mm. are rolled at slower delivery speeds below about 15 m/sec. Here, the frictional resistance imposed by accellerated water cooling is both lessened due to the lower speed of the product, and is safely offset by the greater inherent column strength of the larger diameter products. However, such processes have yet to be applied to modern high speed rod mills, where smaller diameter products exit from the mill at significantly higher mill delivery speeds.